drm: simple_kms_helper: make connector optional at init time

drm_simple_display_pipe_init() pretends to attach a connector
to the display pipe.

In case a drm bridge has to be used, then it's the bridge that
takes care of connectors.

This patch makes the connector parameter optional for
drm_simple_display_pipe_init(), so that a drm bridge could
handle connector by itself later.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1472115874-6219-2-git-send-email-andrea.merello@gmail.com